In wireless communication systems according to WiMAX (Worldwide Interoperability for Microwave Access), data is received and transmitted in units of subframes.
Further, in the wireless communication system according to WiMAX, an OFDMA (Orthogonal Frequency Division Multiple Access) scheme is adopted for multiple access schemes, and a TDD (Time Division Duplex) scheme is adopted for duplex schemes.
The OFDMA scheme is a scheme in which the direction of frequency is divided by subcarriers, the direction of time is divided by time slots and data to be transmitted is allocated a subcarrier and a time slot in which that subcarrier can be used.
The TDD scheme is a scheme in which the same frequency is used for the uplink and the downlink, and uplink subframes and downlink subframes are changed on the time axis.
Furthermore, in the wireless communication system according to WiMAX, individual base stations sometimes form a sector configuration in which a cell is divided into a plurality of sectors by providing directivity for a transmitting antenna in order to increase the number of terminals (Mobile Stations) that can simultaneously communicate with each other.
Here, an exemplary frame structure of an uplink subframe when each base station constructs a three-sector configuration in a wireless communication system according to WiMAX (see FIG. 1) will be described with reference to FIG. 2. In addition, in FIG. 2, suppose that each base station is allocated a 10-MHz bandwidth or 7-MHz bandwidth for a frequency bandwidth.
As shown in FIG. 2, when each base station is allocated a 10-MHz bandwidth or 7-MHz bandwidth, it can use 35 subchannels, each of which has a plurality of subcarriers combined. In this case, each base station distributes and allocates 35 subchannels to individual segments #0, #1, and #2 corresponding to sectors #0, #1, and #2, respectively, in the uplink subframe in order to reduce interference between the sectors (for example, see Patent Literature 1).
For each of segments #0, #1, and #2, the subchannel allocated to each segment is assigned as UL control channels for uplink (UL) control, and then assigned as data channels for uplink data transmission.
The UL control channels include ranging channels used for ranging to adjust the transmission timing, transmission frequency, and transmission electric power at a terminal. More specifically, the ranging channels are formed of an initial/HO ranging channel used for initial and handover (HO) and a periodic ranging/BW request channel used for periodic ranging and bandwidth requesting at regular time intervals. In addition, at least six subchannels are necessary for the ranging channels.
Further, the UL control channels also include a CQICH (Channel Quality Indicator Channel) and an ACK/NACK channel for ACK (Acknowledgment)/NACK (Negative ACK). In addition, both the CQICH and the ACK/NACK channel have variable-length subchannels.